The effects of prolonged sitting with a mild hypercapnic environment on vascular function in older adults

Introduction: Aging is associated with various physiological changes, including declines in vascular endothelial function, increased arterial stiffness, and autonomic dysregulation, all of which increase the risk of developing cardiovascular diseases (CVD). These age-related changes may render older adults particularly vulnerable to factors that can further compound vascular health. Prolonged sitting (PS), defined as sitting for 2 or more hours at a time, is recognized as an independent risk factor for CVD and typically occurs in mild hypercapnic environments (elevated CO 2 concentrations), which can exacerbate vascular dysfunction. However, the effects of PS under mild hypercapnic conditions remain unknown in older adults. This study aimed to comprehensively investigate the cardiovascular response to PS in a mild hypercapnic environment in older adults. A secondary aim was to examine whether active and passive-leg movements exerted a protective effect on vascular function in older adults. Methods: Healthy older adults (n=6, 5 males, 72 ± 3.68 yrs) participated in three experimental visits that consisted of 2.5h of prolonged sitting in a mild hypercapnic environment (CO­­­­­­­­ 2­ = 1500ppm): control (CON, no movement), passive (PASS, passive leg movement), and active (ACT, active leg movement). Popliteal and brachial artery endothelial function was measured using flow-mediated dilation (FMD). Peripheral and central arterial stiffness was assessed using pulse wave velocity. Reoxygenation and metabolic rate, indexes of microvascular function, were assessed using near-infrared spectroscopy. All measurements were taken pre- and post-sitting. Results: Popliteal FMD was reduced in the CON condition compared with both the PASS and ACT (Δ -2.85 ± 0.75%, p<0.0001 vs Δ -0.43 ± 1.28%, P=0.29 vs Δ 1.53 ± 1.12%, P=0.0016, respectively). Brachial FMD was attenuated in the CON condition compared with both the PASS and ACT (Δ -1.86 ± 1.16%, p<0.001 vs Δ -0.41 ± 1.10%, P=0.26 vs Δ 0.99 ± 0.93%, P=0.01, respectively). Reoxygenation rate was slower in the CON condition than the PASS and ACT (Δ -0.39 ± 0.95 %/s, P=0.065 vs Δ -0.01 ± 0.69 %/s, P=0.96 vs Δ -0.14 ± 0.74, P=0.49, respectively). Muscle metabolic rate was only decreased in CON condition (Δ -0.04 ± 0.06 %/s, P=0.03 vs Δ -0.029 ± 0.044 %/s, P=0.07 vs Δ -0.023 ± 0.090 %/s, P=0.15, respectively). No significant changes in central or peripheral arterial stiffness were observed. Conclusion: This preliminary data shows that uninterrupted prolonged sitting with mild hypercapnia further impaired the already compromised macro- and microvascular function of older adults. However, intermittent bouts of active or passive movements were sufficient to preserve leg vascular function during bouts of PS. This research was supported by the University of Nebraska at Omaha Graduate Research and Creative Activity Grant and NASA Nebraska Space Grant (Federal Award #80NSSC20M0112). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.